The subsurface features of a material or structure are frequently of great importance. For example, voids in metal stock may seriusly affect the strength of the material. Similarly, discontinuities in the bond between adjacent layers of a multilayer structure may impair the strength of the structure. In both cases, subsurface features of the particular object being analyzed ultimately determine the object's utility. Thus, detection of the form, size, and location of the subsurface features is important.
In most instances, direct visual observation of subsurface features requires partial destruction of the material or structure of interest. This method of analysis is often counterproductive, rendering the material or structure analyzed useless. Thus, a nondestructive method of analyzing subsurface features of a material or structure is required.
While a number of nondestructive test methods have been developed, each is subject to certain shortcomings. For example, one method of analyzing the subsurface features of an object involves exposing the object of gamma radiation. Because discontinuities or anomalies below the surface respond differently to radiation, the response of the object to the radiation can be used to identify such subsurface features.
This method of analysis, however, suffers from several disadvantages. First, gamma radiation sources are relatively large, making it impractical to produce a portable test apparatus. Second, a test apparatus responding to the transmission of radiation through an object cannot be used in applications where one side of the region of interest is inaccessible. Third, the use of gamma radiation requires that personnel exposed to the radiation during testing be protected, making the process more expensive and less convenient. In addition, gamma radiation analysis may introduce unwanted levels of radiation in the object that remain after testing.
Another method of nondestructively testing an object for subsurface features involves the use of X-rays. Pursuant to this method, the portion of the object containing the subsurface features of interest is placed between an X-ray source and an X-ray-sensitive film. The surface of the object is then exposed to X-rays. Because the subsurface features influence the X-rays received by the film, the developed film produces an image of the object that can be used to identify the features.
One problem with this method of analysis is that it may be difficult to produce X-rays having enough energy to penetrate many objects to the degree necessary to provide a clear image. For example, the analysis of heavy sheet steel may require a relatively high-energy X-ray source. X-ray analysis also has the disadvantage of requiring that film be placed behind the material or structure to be analyzed. Finally, the process is relatively expensive, particularly when the features beneath a large surface are to be analyzed, because a correspondingly large quantity of film is required.
Ultrasonic test methods have been proposed that overcome some of the problems involved with the use of gamma radiation and X-rays. Ultrasonic analysis involves the exposure of the surface to ultrasonic energy. The response of the object to the excitation is then used to indicate the presence of subsurface features. This method is safer, more convenient, and less expensive than those using gamma radiation and X-rays, but may produce erroneous signals. For example, when the subsurface features of an inspected object include layers of the same or different materials, reflections produced at the interface between the layers may produce an indication of a nonexistent anomaly, such as an air gap.
One nondestructive test method that overcomes these problems is thermography. In principle, thermographic testing involves the exposure of the analyzed surface to even heating. Variations in the thermal conductivity of features below the surface then allow heat to flow away from the surface more rapidly in some places than others, establishing temperature gradients along the surface that provide an indication of the subsurface features of the object. For example, if the structure being examined is made of a material having a relatively high thermal conductivity, like steel, the temperature of the surface will be lower adjacent thicker portions of the material. Similarly, if the material contains voids, the relatively poor conductivity of the air limits heat flow from the surface, resulting in a warm spot on the surface adjacent the void. Thus, temperature patterns on the surface can be used to identify subsurface features.
A number of methods and apparatus have been devised for the nondestructive, thermographic testing of subsurface features. For example, in U.S. Pat. No. 2,260,168 (McNutt), an early form of thermographic testing is illustrated. There, a row of torches acting as a heat source is passed over a metal surface at a uniform speed. While the reference suggests that a plurality of temperature-responsive devices can be positioned following the heat source, in the preferred embodiment the temperature of the heat source is sufficiently high to produce discoloration of the surface adjacent subsurface voids. The arrangement disclosed by McNutt, however, introduces a number of problems. For example, many nonmetallic surfaces may be incapable of producing the requisite discoloration in response to subsurface features. In addition, the relatively high temperatures required to produce discoloration, and thus identify defects, may be destructive when used with many materials. Finally, material analyzed in this manner must be allowed to cool before being handled by test personnel.
A second thermographic test apparatus is illustrated in U.S. Pat. No. 3,206,603 (Mauro). Pursuant to this arrangement, a radiant energy source or induction heating coil supplies heat to the surface of the material being analyzed. As the material being analyzed is passed under the apparatus, infrared energy radiated from its surface is received by an optical system and directed to a rotating chopper mirror, which alternatively reflects the infrared energy to a pair of thermistor bolometers. Thus, the bolometers receive radiation from alternating portions of the surface. A difference in the outputs of the two bolometers indicates a temperature gradient on the surface and thus, the presence of a subsurface anomaly. This arrangement is relatively complex and, when a large portion of the surface is to be analyzed, the use of a single optical path and pair of bolometers requires an inconveniently large displacement between the apparatus and the surface.
A third arrangement for thermographically testing objects is illustrated in U.S. Pat. No. 3,222,917 (Roth). A thermal pulse is applied to a first region of the object under test. A pickup transducer, such as a thermistor, pyroelectric element, or thermocouple, is positioned adjacent a second region of the object. The pickup transducer produces an output dependent upon the transient response of the object to the thermal pulse. The time-dependent amplitude of the response varies if there are flaws present between the region where the thermal pulse was applied and the region adjacent the transducer. This method appears somewhat impractical for use in locating flaws in large objects because the transient response of numerous incremental portions of the object would have to be determined, each taking some time to complete. Thus, inspection of the entire object would be quite time consuming.
A fourth method of thermographic flaw detection is described in U.S. Pat. No. 3,462,602 (Apple). There, rolled sheet steel stock is inspected while it is still hot. A pair of radiometers scan the rolled stock at two spaced-apart locations. The output of each radiometer is a function of the temperature of the surface area viewed. The two outputs are fed to a differential amplifier that produces an output whose magnitude is dependent upon the difference between the two radiometer outputs. Thus, if the temperature of the sheet stock is uniform, the output of the differential amplifier is zero. If the temperature of the two regions viewed by the radiometers is different, however, the differential amplifier produces a nonzero output. a trigger circuit signals the presence of a defect when the output of the differential amplifier exceeds a predetermined level.
There are several disadvantages with the Apple arrangement. First, radiometers must be displaced a relatively large distance from the surface if more than a small point on the surface is to be observed. In addition, two radiometers are required to determine the presence of a flow in one portion of the workpiece. Third, if small defects are to be located in a relatively large object, the distance between the radiometer observation points must be relatively small and a substantial amount of time is required to sweep the entire surface.
In light of the foregoing discussion, it would be desirable to develop a method of, and apparatus for, nondestructively locating and identifying nearsurface features of an object when only one surface of the object is accessible. Such a system should be capable of rapidly locating and identifying relatively small features below the surface. In addition, the apparatus should be compact, relatively simple, and inexpensive.